Documentation improvement.

I have just been bitten by this.

While it's really related to select2, due to the fact that django-select2 adds his bits of abstraction on top of select2, I wasted quite some time making sure nothing was wrong with django-select2 in the first place...

Hope it can help others, or help me if I happen to forget about it ;)
Merge pull request #104 from johnraz/patch-1

Documentation improvement.